Lean body mass is a major determinant of levothyroxine dosage in the treatment of thyroid diseases

Total body weight is usually employed to calculate the amount of L-T 4 to be administered in patients with thyroid diseases. The aim of this study was to evaluate the effect off body composition on L-T4 requirements. Body composition was assessed by dual energy x-ray absorptiometry in 75 patients on TSH-suppressive L-T4 therapy after conventional thyroid ablation for differentiated cancer. The mean daily dose of L-T 4 was lower in normal-weight (127.5 ± 21.3 μg/d) vs. overweight (139.4 ± 24.5) and obese (151.3 ± 29.1) subjects. There was a much stronger association between the L-T4 dosage and lean body mass (P < 0.001, r = 0.667) compared with fat mass (P = 0.023, r = 0.26). Measurement of regional tissue composition showed peripheral lean mass as the best correlate with the dose of L-T4 (r = 0.679, P < 0.001) whereas no correlation was observed with peripheral fat mass. In conclusion, individual L-T4 requirements are dependent on lean body mass. Age- and gender-related differences in L-T4 needs reflect different proportions of lean mass over the total body weight. An estimate of lean mass may be helpful to shorten the time required to attain a stable dose of L-T 4, particularly in subjects with high body mass index values that may be due either to increased muscular mass or to obesity.